Ceramic products of a honeycomb shape, or ceramic honeycomb structures, i.e., cellular ceramic bodies, have been made by preparing a ceramic green body through mixing of ceramic materials with water and various carbonaceous materials, including extrusion and forming aids to form a plasticized batch, forming the body into a honeycomb-shaped ceramic green body through extrusion of the plasticized batch, and finally firing the honeycomb-shaped ceramic green body in a firing furnace at a predetermined temperature.
Extrusion and forming aids used in the above firing of the honeycomb structure include, specifically, organic binders and plasticizers and lubricants, such as methylcelloluse, carboxymethlcellulose, polyvinyl alcohol, alkali stearates and the like. Furthermore, other carbonaceous materials such as graphite have been included in the batch as a pore-forming agent.
It is known that the carbonaceous material release or the decomposition of the carbonaceous material, is an oxidation or exothermic reaction which releases large amounts of heat. Initially the exothermic reaction occurs at the skin or outer portion of the part, resulting in an initial thermal differential whereby the outer portion of the ceramic body is hotter than the core. Subsequently, the skin or outer portion exothermic reaction dies down, and the exothermic reaction region moves into the interior of the ware. Because typical substrates are comprised of ceramic materials, for example cordierite, which are good insulators, and exhibit a cellular structure comprising numerous channels, difficulties are encountered in effectively removing, either by conduction or convection, the heat from the ceramic body. Additionally, due to the cellular structure there is considerable surface area to promote binder reaction with the O.sub.2 in the firing atmosphere, thus exacerbating this interior exothermic effect. As such, during the carbonaceous material release, the ceramic body exhibits either a positive or negative thermal differential; i.e., the core of the ceramic body exhibiting either a higher or lower temperature than that of the ceramic at/near the surface. This exothermic reaction, which occurs in the 100 to 600.degree. C. temperature range for carbonaceous materials such as an organic binder or the like, or in the 500-1000.degree. C. temperature range if the body contains, for example, graphite, causes a significant temperature differential between the inside and outside of the part. This temperature differential in the part creates stresses in the ceramic body which may result in cracking of the part. This phenomenon is particularly true for large cellular ceramic parts or parts containing large amounts of organic materials.
Techniques for controlling and inhibiting the thermal differential and resultant crack development are well known. One technique involves reducing burner flame temperature by using excess air for burner combustion, resulting in a reduced flame to product temperature gradient and corresponding slower ware heating rates. However, the high excess air yields an undesirably high percentage oxygen-containing atmosphere that reacts with the organics thereby accelerating release and increasing the internal exothermic reaction. As such, minimization of the thermal differential which develops during organic release, must be accomplished through very slow firing schedules or, alternatively, firing schedules which are carefully matched to the particular ware in the kiln.
Use of atmosphere control in periodic-type kilns to affect carbonaceous material release is generally known. See, for example, U.S. Pat. Nos., 4,404,166 (Wiech, Jr.), 4,474,731 (Brownlow et al.), 4,661,315 (Wiech Jr. et al.) and 4,927,577 (Ohtaka et al.). Although these methods have been shown to be effective enough for use in periodic-type kilns, they are not generally considered to be effective in tunnel kilns due to the considerable influx of ambient air (20.9% oxygen) into the firing atmosphere.
The use of pulse firing technology as a substitute for proportional firing has also been disclosed as a method for controlling and inhibiting thermal gradients in periodic kilns. Pulse firing involves the use of either high fire or low fire burner output conditions only, and produces low heating rates without the use of considerable amounts of excess air (oxygen); see, for example Eur. Pat. Appl. No. 0 709 638 which discloses a method of firing ceramic formed bodies using a furnace having burners which alternate from a high to a low output firing state. Although the use of this firing technology has been somewhat effective in periodic kilns, resulting in a reduction in the incidences of cracking, this pulse firing technique poses difficulties when used in tunnel kilns. Due to the open nature of tunnel kilns it is necessary to control the ambient air ingress into the organic release zones of the kiln by other means.
Therefore, an object of the invention is to solve the above-mentioned problems of the prior art by providing an improved tunnel kiln for use in firing high-quality crack-free ceramic honeycomb structural bodies.